Remove an element from a Bash array

Question

I need to remove an element from an array in bash shell  Generally I d simply do   array        array   lt element to remove gt       Unfortunately the element I want to remove is a variable so I can t use the previous command  Down here an example   array   pluto  array   pippo  delete  pluto  array    array     delete    - gt  but clearly doesn t work because of      Any idea

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Here s a one-line solution with mapfile     mapfile -d    0  -t arr  lt   lt  printf   s 0     arr        grep -Pzv   lt regexp gt      Example     arr   Adam   Bob   Claire    n  Smith   David   Eve   Fred      echo  Size     arr     Contents    arr       Size  6 Contents  Adam Bob Claire Smith David Eve Fred    mapfile -d    0  -t arr  lt   lt  printf   s 0     arr        grep -Pzv   Claire nSmith       echo  Size     arr     Contents    arr       Size  5 Contents  Adam Bob David Eve Fred   This method allows for great flexibility by modifying exchanging the grep command and doesn t leave any empty strings in the array

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The following works as you would like in bash and zsh     array  pluto pippo    delete pluto   echo   array     delete  pippo   array      array     delete      Quotes when working with strings   If need to delete more than one element         delete  pluto pippo  for del in   delete     do    array     array     del     Quotes when working with strings done   Caveat  This technique actually removes prefixes matching  delete from the elements  not necessarily whole elements   Update  To really remove an exact item  you need to walk through the array  comparing the target to each element  and using unset to delete an exact match   array  pluto pippo bob  delete  pippo  for target in    delete       do   for i in     array       do     if      array i      target     then       unset  array i       fi   done done   Note that if you do this  and one or more elements is removed  the indices will no longer be a continuous sequence of integers     declare -p array declare -a array   0   pluto   2   bob     The simple fact is  arrays were not designed for use as mutable data structures  They are primarily used for storing lists of items in a single variable without needing to waste a character as a delimiter  e g   to store a list of strings which can contain whitespace    If gaps are a problem  then you need to rebuild the array to fill the gaps   for i in     array       do     new array       array i      done array     new array       unset new array

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Here s a  probably very bash-specific  little function involving bash variable indirection and unset  it s a general solution that does not involve text substitution or discarding empty elements and has no problems with quoting whitespace etc    delete ary elmt       local word  1        the element to search for  amp  delete   local aryref   2       a necessary step since      2      is a syntax error   local arycopy      aryref      create a copy of the input array   local status 1   for    i      arycopy     - 1  i  gt   0  i--     do   iterate over indices backwards     elmt   arycopy  i           elmt     word     amp  amp  unset   2  i    amp  amp  status 0   unset matching elmts in orig  ary   done   return  status   return 0 if something was deleted  1 if not    array  a 0 0 b 0 0 0 c 0 d e 0 0 0  delete ary elmt 0 array for e in    array       do   echo   e  done    prints  a   b   c   d  in lines   Use it like delete ary elmt ELEMENT ARRAYNAME without any   sigil  Switch the     word for     word  for prefix matches  use   elmt         word    for case-insensitive matches  etc   whatever bash    supports   It works by determining the indices of the input array and iterating over them backwards  so deleting elements doesn t screw up iteration order   To get the indices you need to access the input array by name  which can be done via bash variable indirection x 1  varname x  echo    varname    prints  1    You can t access arrays by name like aryname a  echo     aryname      this gives you an error  You can t do aryname a  echo     aryname       this gives you the indices of the variable aryname  although it is not an array   What DOES work is aryref  a      echo     aryref    which will print the elements of the array a  preserving shell-word quoting and whitespace exactly like echo    a       But this only works for printing the elements of an array  not for printing its length or indices  aryref   a     or aryref   a     or      aryref   or      aryref    they all fail    So I copy the original array by its name via bash indirection and get the indices from the copy  To iterate over the indices in reverse I use a C-style for loop  I could also do it by accessing the indices via    arycopy     and reversing them with tac  which is a cat that turns around the input line order   A function solution without variable indirection would probably have to involve eval  which may or may not be safe to use in that situation  I can t tell

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You could build up a new array without the undesired element  then assign it back to the old array  This works in bash   array  pluto pippo  new array    for value in    array      do         value    pluto     amp  amp  new array    value  done array     new array       unset new array   This yields   echo    array      pippo

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What I do is   array    echo  array   tr       n    sed   itemtodelete d      BAM  that item is removed

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To expand on the above answers  the following can be used to remove multiple elements from an array  without partial matching   ARRAY  one two onetwo three four threefour  one six   TO REMOVE  one four   TEMP ARRAY    for pkg in    ARRAY       do     for remove in    TO REMOVE       do         KEEP true         if      pkg       remove      then             KEEP false             break         fi     done     if   KEEP   then         TEMP ARRAY     pkg       fi done ARRAY     TEMP ARRAY       unset TEMP ARRAY   This will result in an array containing   two onetwo three threefour  one six

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There is also this syntax  e g  if you want to delete the 2nd element    array     array    0 1      array    2      which is in fact the concatenation of 2 tabs  The first from the index 0 to the index 1  exclusive  and the 2nd from the index 2 to the end

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This answer is specific to the case of deleting multiple values from large arrays  where performance is important   The most voted solutions are  1  pattern substitution on an array  or  2  iterating over the array elements  The first is fast  but can only deal with elements that have distinct prefix  the second has O n k   n array size  k elements to remove  Associative array are relative new feature  and might not have been common when the question was originally posted   For the exact match case  with large n and k  possible to improve performance from O nk  to O n klog k    In practice  O n  assuming k much lower than n  Most of the speed up is based on using associative array to identify items to be removed   Performance  n-array size  k-values to delete   Performance measure seconds of user time     N     K     New seconds  Current seconds   Speedup  1000   10     0 005        0 033             6X 10000   10     0 070        0 348             5X 10000   20     0 070        0 656             9X 10000    1     0 043        0 050             -7    As expected  the current solution is linear to N K  and the fast solution is practically linear to K  with much lower constant  The fast solution is slightly slower vs the current solution when k 1  due to additional setup   The  Fast  solution  array list of input  delete list of values to remove           declare -A delk         for del in    delete        do delk  del  1   done                   Tag items to remove  based on         for k in     array        do                      delk   array  k   -      amp  amp  unset  array k           done                   Compaction         array     array         Benchmarked against current solution  from the most-voted answer       for target in    delete       do         for i in     array       do             if      array i      target     then                 unset  array i               fi         done     done     array     array

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In ZSH this is dead easy  note this uses more bash compatible syntax than necessary where possible for ease of understanding      I always include an edge case to make sure each element   is not being word split  start  one two three  four 4  five  work       start    idx 2 val   work idx      How to remove a single element easily    Also works for associative arrays  at least in zsh  work  idx      echo  Array size went down by one         work -eq      start - 1       amp  amp  echo  OK   echo  Array item   val  is now gone       -z   work  r  val       amp  amp  echo OK  echo  Array contents are as expected    wanted     start    0 1      start    2           j    wanted             j    work          amp  amp  echo  OK   echo  -- array contents  start --  print -l -r --  --   start elements       start  echo  -- array contents  work --  print -l -r --  --   work elements     work        Results   Array size went down by one  OK Array item two is now gone  OK Array contents are as expected  OK -- array contents  start -- -- 5 elements one two three four 4 five -- array contents  work -- -- 4 elements one three four 4 five

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How about something like   array  one two three  array t     array       delete one array    array t    delete      unset array t

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bin bash  echo    define array with six elements  arr  zero one two three  four 4  five   echo    unset by index  0  unset -v  arr 0   for i in    arr      do echo  arr  i    arr  i     done  arr delete by content       value to delete         for i in    arr      do                      arr  i        1     amp  amp  unset -v  arr  i           done            echo    unset in global variable where value  three  arr delete by content three for i in    arr      do echo  arr  i    arr  i     done  echo    rearrange indices  arr      arr        for i in    arr      do echo  arr  i    arr  i     done  delete value       value arrayelements     returns array decl          local e val  1  new     shift         for e in         do     val       e     amp  amp  new     e    done         declare -p new sed  s                       echo    new array without value  two  declare -a arr    delete value two    arr        for i in    arr      do echo  arr  i    arr  i     done  delete values       arraydecl values     returns array decl   keeps indices          declare -a arr   1   local i v  shift         for v in         do                  for i in    arr      do                             v       arr  i       amp  amp  unset -v  arr  i                   done         done         declare -p arr sed  s                      echo    new array without values  one five  keep indices   declare -a arr    delete values    declare -p arr sed  s              one five   for i in    arr      do echo  arr  i    arr  i     done    new array without multiple values and rearranged indices is left to the reader

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This is a quick-and-dirty solution that will work in simple cases but will break if  a  there are regex special characters in  delete  or  b  there are any spaces at all in any items   Starting with   array   pluto  array   pippo  delete  pluto    Delete all entries exactly matching  delete   array   echo  array   fmt -1   grep -v     delete      fmt -999999     resulting in echo  array -  pippo  and making sure it s an array  echo  array 1  -  pippo  fmt is a little obscure  fmt -1 wraps at the first column  to put each item on its own line   That s where the problem arises with items in spaces    fmt -999999 unwraps it back to one line  putting back the spaces between items   There are other ways to do that  such as xargs   Addendum  If you want to delete just the first match  use sed  as described here   array   echo  array   fmt -1   sed  0     delete      d      fmt -999999

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If anyone finds themselves in a position where they need to remember set -e or set -x values and be able to restore them  please check out this gist which uses the first array deletion solution to manage it s own stack   https   gist github com kigster 94799325e39d2a227ef89676eed44cc6

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Partial answer only  To delete the first item in the array  unset  array 0     To delete the last item in the array  unset  array -1

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POSIX shell script does not have arrays   So most probably you are using a specific dialect such as bash  korn shells or zsh   Therefore  your question as of now cannot be answered   Maybe this works for you   unset array  delete

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To avoid conflicts with array index using unset - see https   stackoverflow com a 49626928 3223785 and https   stackoverflow com a 47798640 3223785 for more information - reassign the array to itself  ARRAY VAR    ARRAY VAR           bin bash  ARRAY VAR  0 1 2 3 4 5 6 7 8 9  unset ARRAY VAR 5  unset ARRAY VAR 4  ARRAY VAR    ARRAY VAR      echo   ARRAY VAR     A LENGTH    ARRAY VAR     for    i 0  i lt       A LENGTH -1     i        do     echo        echo  INDEX -  i      echo  VALUE -   ARRAY VAR  i    done  exit 0    Ref   https   tecadmin net working-with-array-bash-script

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This is the most direct way to unset a value if you know it s position     array  one two three    echo    array     3   unset  array 1     echo   array     one three   echo    array     2

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Using unset  To remove an element at particular index  we can use unset and then do copy to another array  Only just unset is not required in this case  Because unset does not remove the element it just sets null string to the particular index in array    declare -a arr   aa   bb   cc   dd   ee   unset  arr 1   declare -a arr2    i 0 for element in    arr      do     arr2  i   element         i   done echo    arr      echo  1st val is   arr 1    2nd val is   arr 2    echo    arr2      echo  1st val is   arr2 1    2nd val is   arr2 2      Output is  aa cc dd ee 1st val is   2nd val is cc aa cc dd ee 1st val is cc  2nd val is dd   Using   lt idx gt   We can remove some set of elements using   lt idx gt  also  For example if we want to remove 1st element we can use  1 as mentioned below   declare -a arr   aa   bb   cc   dd   ee   arr2     arr    1    echo    arr2      echo  1st val is   arr2 1    2nd val is   arr2 2      Output is   bb cc dd ee 1st val is cc  2nd val is dd

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http   wiki bash-hackers org syntax pe substring removal    PARAMETER PATTERN    remove from beginning   PARAMETER  PATTERN    remove from the beginning  greedy match   PARAMETER PATTERN    remove from the end   PARAMETER  PATTERN    remove from the end  greedy match  In order to do a full remove element  you have to do an unset command with an if statement   If you don t care about removing prefixes from other variables or about supporting whitespace in the array  then you can just drop the quotes and forget about for loops  See example below for a few different ways to clean up an array  options   quot foo quot   quot bar quot   quot foo quot   quot foobar quot   quot foo bar quot   quot bars quot   quot bar quot      remove bar from the start of each element options   quot   options      quot bar quot   quot     options   quot foo quot   quot  quot   quot foo quot   quot foobar quot   quot foo bar quot   quot s quot   quot  quot      remove the complete string  quot foo quot  in a for loop count    options     for   i   0  i  lt  count  i      do    if    quot   options i   quot     quot foo quot      then       unset  options i      fi done   options     quot  quot     quot foobar quot   quot foo bar quot   quot s quot   quot  quot      remove empty options   note the count variable can t be recalculated easily on a sparse array for   i   0  i  lt  count  i      do      echo  quot Element  i     options i    quot     if   -z  quot   options i   quot      then       unset  options i      fi done   options   quot foobar quot   quot foo bar quot   quot s quot      list them with select echo  quot Choose an option  quot  PS3  Option    select i in  quot   options     quot  Quit  do     case  i in         Quit  break              echo  quot You selected   quot  i  quot  quot         esac  done  Output Choose an option  1  foobar 2  foo bar 3  s 4  Quit Option    Hope that helps

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Actually  I just noticed that the shell syntax somewhat has a behavior built-in that allows for easy reconstruction of the array when  as posed in the question  an item should be removed     let s set up an array of items to consume  x    for    i 0  i lt 10  i       do     x     i   done    here  we consume that array  while       x         do     i      RANDOM      x            echo    x i       x          x     x    0 i      x    i 1    done   Notice how we constructed the array using bash s x     syntax   You could actually add more than one item with that  the content of a whole other array at once

[arrays] Remove an element from a Bash array

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